Mishnah 7 in chapter 4 of Maseches Nazir also deals with a halachah leMoshe miSinai in the laws of nezirus: a person's ability to use money that his father had set aside for his own nezirus for his own nezirus. In this mishnah we will learn under what circumstances this is possible.
The text of the mishnah:
"Ha'ish megale'ach al nezirus aviv, ve'ein ha'ishah megalachas al nezirus aviha" - a man may shave his head and bring the offerings of his nezirus from the money that his father set aside for his own nezirus, whereas a woman may not use her father's money for the offerings of her nezirus.
The case that is not considered "shaving on his father's nezirus":
How so? The mishnah explains: "Mi shehayah aviv nazir, vehifrish ma'os al nezirso umes" - a person's father was a nazir, he set aside unspecified money without designating which coins were meant for which offering, and he died. The money is specifically set aside without designation, for had he designated money for the chatas, it would turn out that the designation came too late: once the owner of the offering dies, it becomes a chatas whose owner died, whose law is that it must be left to die. For this reason the money was not designated, and it was in this state that the father died.
The son says: "Hareini nazir al menas she'agale'ach al ma'os abba" - I hereby accept nezirus upon myself on condition that I will be able to fulfill the shaving and the offerings that accompany it from the money that my father set aside.
"Amar Rabbi Yosei: Harei eilu yipelu linedavah, ein zeh megale'ach al nezirus aviv" - Rabbi Yosei rules that the condition is of no effect. The law of that money follows the rule we learned: when a nazir dies, his money falls to nedavah - to the communal donation fund, from which voluntary offerings are brought. This, then, is not an example of a person shaving on his father's nezirus money.
Who, then, is the one who shaves on his father's nezirus?
"Mi shehayah hu ve'aviv nezirim, vehifrish aviv ma'os stumim linezirso umes - zehu shemegale'ach al nezirus aviv" - already in the father's lifetime both of them, father and son, were nezirim; the father set aside unspecified money for his nezirus and died. In this case the son may use his father's money for the offerings of his nezirus. But if at the time of his father's death the son was not yet a nazir, he may not do so.
In summary: in this mishnah we learned a halachah leMoshe miSinai, that a man shaves on his father's nezirus while a woman does not shave on her father's nezirus. We saw that the money is set aside without designation, because of the law of a chatas whose owner died; and we learned Rabbi Yosei's distinction - a son who accepted nezirus upon himself after his father's death on condition that he shave on his father's money gains nothing from that money, and it falls to nedavah, whereas one who was already a nazir during the lifetime of his father the nazir, who set aside unspecified money and died, is the one who shaves on his father's nezirus.